The present invention relates to a field effect transistor which uses liquid electrolyte as a gate and a hydrogen-terminated surface of diamond as a channel.
An undoped hydrogen-terminated diamond thin film formed by a microwave plasma CVD method has been known to have a p-type conductive layer formed on its surface, regardless of whether the diamond thin film is monocrystalline or polycrystalline. Thus far, the present inventors have fabricated a field effect transistor (FET) in which deposition of metal or insulating substance on the gate portion is avoided through utilization of such a surface conductive layer and have tested the operation of the FET within an electrolytic aqueous solution (Ken-ichi Kitatani, et al., Proceedings of the 46th Applied Physics Joint Lecture Meeting, 30a-P7-22 (1999) pp. 628).
Further, an electric double layer is considered to be formed at the interface between a surface conductive layer of a diamond in an electrolytic solution and the electrolyte (Takeshi Hosomi, et al., Abstracts of the 13th Diamond Symposium Lecture Meeting, 115 (1999) pp. 36).
The present inventors have tackled development of an ISFET biosensor in which an ion sensitive field effect transistor (ISFET) is used as a transducer, and an enzyme is used as a molecular recognizing substance, and have found that such an ISFET biosensor can be used as a micro-sensor for clinical test, industrial measurement, and environmental measurement, with possible future application to a body-embedable sensor.
However, at present, such an ISFET biosensor has a practical problem in that even when the molecular recognizing substance functions properly and an intended molecule is selected, operation of the ISFET itself becomes unstable due to a substance invading from outside the sensor. This unstable condition is caused by a great variation in threshold voltage attributable to generation of an interface level, or presence of ions at the oxide film/Si interface, which is the most delicate portion within an Si MOSFET. When a sensor is used, its surface is unavoidably exposed to the atmosphere. In the case of an ion sensor or biosensor, even when a protective layer is used, a considerably large number of undesired molecules or ions invade the oxide film/Si interface as compared with the case of LSIs. Therefore, creation of a chemically strong semiconductor surface is indispensable for development of ISFET biosensors.
Incidentally, a hydrogen-adsorbed surface (hydrogen-terminated surface) of diamond is stable up to 700xc2x0 C. in vacuum, and up to 300xc2x0 C. in the atmosphere; is chemically inert even in liquid; and, near room temperature, does not cause structure change even when exposed to strong acid or strong alkali. In addition, a p-type conductive layer is formed in the vicinity of the hydrogen-terminated diamond surface.
On the basis of the above-described research, the present inventors have developed an FET in which a liquid electrolyte is used as a gate.
In view of the foregoing, an object of the present invention is to provide a field effect transistor which uses a liquid electrolyte as a gate and which operates stably in the liquid electrolyte.
In order to achieve the above object, the present invention provides the following.
[1] A field effect transistor characterized by comprising a channel formed of a portion of a hydrogen-terminated surface of a diamond, the portion being exposed to the outside between a source electrode and a drain electrode; and a gate formed of a liquid electrolyte in contact with the exposed portion of the hydrogen-terminated surface of the diamond.
[2] A field effect transistor as described in [1] above, characterized in that the diamond is formed of an undoped, hydrogen-terminated, monocrystalline or polycrystalline diamond thin film.
[3] A field effect transistor as described in [1] above, characterized in that the channel is a p channel.
[4] A field effect transistor as described in [1] above, characterized in that the p channel can be pinched off.
[5] A field effect transistor as described in [1] above, characterized in that the p channel is of a normally off type.
[6] A field effect transistor as described in [1] above, characterized in that the hydrogen-terminated surface of the diamond has a wide potential window, and the field effect transistor operates accurately within the range of the potential window.
[7] A field effect transistor as described in [1] above, characterized in that the liquid electrolyte is an alkaline solution.
[8] A field effect transistor as described in [7] above, characterized in that the liquid electrolyte is a KOH aqueous solution.
[9] A field effect transistor as described in [1] above, characterized in that the liquid electrolyte is an acidic solution.
[10] A field effect transistor as described in [1] above, characterized by having a threshold voltage which exhibits low dependence on the pH of the liquid electrolyte.
[11] A field effect transistor as described in [1] above, characterized by having a threshold voltage which is not affected by the environment.